1. Field of the Invention
This invention relates to airborne look-down Doppler radars and, more particularly, to a system and method for extracting rotor features from a Doppler radar to track hovering helicopters.
2. Description of the Related Art
The significance of the helicopter threat continues to grow as U.S. interests are challenged in austere, worldwide locations by conventional forces and terrorist organizations employing unconventional, asymmetric means of warfare. Without an ability to field a conventional air force, adversaries place greater emphasis on the wide range of missions which can be conducted by a relatively inexpensive helicopter force. The helicopter is a particularly difficult target for air- or surface-launched missiles that use airborne look-down Doppler radars. Speeds that range from over 200 knots to a hover, extremely low-altitude flight, an ability to terrain mask or hide using terrain features and an ability to employ a wide variety of lethal ordnance are some of a helicopter's challenging characteristics. Ground-launched missiles that employ look-up thermal detection are a significant threat to hovering or slow-moving helicopters. However, such capability is not always available in the theater of operations when and where they are needed. Furthermore, shoulder-launched missiles are less effective against fast-moving helicopters. At present, the ability to effectively counter helicopters is less than robust.
As shown in FIG. 1a, a surface- or air-launched missile 10 employs a look-down Doppler radar (“seeker 11”) to transmit an electromagnetic signal 12, typically an X-band radio wave, towards a hostile helicopter 14 and processes the return signal to detect, identify, and track the helicopter. The seeker is typically a pulsed radar which uses range gates and Doppler filters to observe targets at different ranges and Doppler. Alternately, a continuous-wave (CW) radar might be used. A conventional CW radar only provides Doppler information, but it can be modified to provide range information.
The clutter area Ac for the range gate in which the helicopter is located is given by:
      A    c    ≈      R    ⁢                  ⁢          θ              3        ⁢                                  ⁢        dB              ⁢                  c        ⁢                                  ⁢        τ            2        ⁢    sec    ⁢                  ⁢          ψ      g      Where R is the range from the seeker to earth along a main axis of beam 12, θ3DB is the antenna's 3 dB beam width, c is the speed of light, τ is the pulse width, and ψg is the incident grazing angle measured from the earth's surface to the main axis of beam 12.
The signal-to-clutter ratio of the electromagnetic return from the clutter area Ac including the helicopter is given by:
      Signal    ⁢          -        ⁢    to    ⁢          -        ⁢    Clutter    ⁢                  ⁢    Ratio    ∝                    σ        helicopter            ⁢      cos      ⁢                          ⁢              ψ        g                            σ        ground            ⁢              θ                  3          ⁢                                          ⁢          dB                    ⁢      Rc      ⁢                          ⁢      τ      Where σhelicopter is the helicopter radar cross-section (RCS) and σground are the ground reflection coefficients at the incident grazing angle.
Target detection and identification is based on analyzing the properties of a received signal. These properties (from the easiest to most difficult) are: signal amplitude, target angle, target range, target speed along the line-of-sight (Doppler shift), target speed across the line-of-sight, and target shape. As shown in FIG. 1b, a hovering or slow-moving helicopter is very difficult to intercept with a look-down radar because its body Doppler 16 has merged with ground clutter 18. The Doppler extent 20 (width of the Doppler spectrum) of ground clutter is determined by the motions of the seeker and the illuminating aperture.
Conventional seekers usually sample return signals and attempt to exclude returns from the ground or other undesirable returns. A target can be detected when the return samples exceed the system noise by a sufficient margin. For example, the seeker maintains a desired constant false-alarm rate (CFAR) by changing the false-alarm threshold T1 to an optimum value that varies over time. Once a possible target is detected, the seeker uses more-sophisticated processing to try to classify the body Doppler and determine the range and range-rate of the target. Once a target is positively identified, its range and range-rate are passed to a tracking processor which guides the missile during the terminal guidance phase to impact the target.
If the target is an airplane or fast-moving helicopter, its body Doppler 16 is shifted away from ground clutter, and its signal-to-clutter ratio is high enough for standard techniques to be effective. A hovering helicopter's body Doppler, however, has merged with clutter and only the return 26 of its rotor assembly 28 extends outside of clutter (due to the rotation of the rotor assembly). Even if the helicopter were moving, different flight geometries could put the helicopter's body Doppler within the clutter region. If a seeker tries to estimate the range and range-rate of the rotor return, it will find conflicting range-rate measurements since the rotor return constantly changes with time and scintillates (both in amplitude and angle). Thus, the seeker will disregard a majority of the helicopter's rotor return, and the rotor return samples will not be used to classify the potential target as a helicopter.
There remains a need for a robust technique for detecting and classifying hovering and slow-moving helicopters that is compatible with the existing base of Doppler radars.